Oxidative addition of silanes R3SiH to the unsaturated cluster [Os3(µ-H){µ3-Ph2PCH2PPh(C6H4)}(CO)8]: Evidence for reversible silane formation in the dynamic behaviour of [Os3(µ-H)(SiR3)(CO)9(µ-dppm)]

Oxidative addition of the silanes R3SiH (R3= Ph3, Et3, EtMe2) to the unsaturated cluster [Os3(µ-H){µ3-Ph2PCH2PPh(C6H4)}(CO)8] leads to the saturated clusters [Os3(µ-H)(SiR3)(CO)9(µ-dppm)](SiR3= SiPh31, SiEt32 and SiEtMe23) and the unsaturated clusters [Os3(µ-H)2(SiR3){µ3-Ph2PCH2PPh(C6H4)}(CO)7](SiR3= SiPh34, SiEt35 and SiEtMe26). Structures are based on spectroscopic evidence and a XRD structure of [Os3(µ-H)(SiPh3)(CO)9(µ-dppm)]1 in which all non-CO ligands are coordinated equatorially and the hydride and the silyl groups are mutually cis. From variable-temperature 1H NMR spectra of the SiEt3 compound 2, exchange of the P nuclei is clearly apparent. Simultaneous migrations of the SiEt3 group and of the hydride from one Os–Os edge to another generate a time-averaged mirror plane in the molecule. VT 1H NMR spectra of the somewhat less bulky compound [Os3(µ-H)(SiMe2Et)(CO)9(µ-dppm)]3 have been analysed. Two isomers 3a and 3b are observed with the hydride ligand located on different Os–Os edges. Synchronous migration of the hydride and SiMe2Et groups is faster than the observed interconversion of isomers which occurs by hydride migration alone. The synchronous motion of H and SiR3only occurs when these ligands are mutually cis as in the major isomer 3a and we propose that this process requires the formation of a transient silane complex of the type [Os3(2-HSiR3)(CO)9(µ-dppm)]. Turnstile rotation within an Os(CO)3(2-HSiR3) group leads to the observed exchange within the major isomer 3a without exchange with the minor isomer. This process is not observed for the minor isomer 3b because the hydride and the silyl group are mutually trans. Protonation to give [Os3(µ-H)2(SiR3)(CO)9(µ-dppm)]+ totally suppresses the dynamic behaviour because there are no edge vacancies.